11 October 2012

Two disturbances to watch across the basin

The first disturbance is an easterly wave that left the African coast back on October 5, and has been making its way westward.  It's now centered near 11N 56W (about 340 miles east of Grenada in the southern Lesser Antilles) and has an embedded 1007mb Low.

Fortunately for North American interests, models are in good agreement on this system turning to the north very soon, probably passing east of Puerto Rico (rather similar to Jose 1999). One exception is the ECMWF global model, which has a more northwest track, bring it over Puerto Rico, then the Bahamas, THEN recurves it on Monday over the Bahamas before it reaches Florida.

As far as intensity goes, it has a good chance at becoming a Depression and then Tropical Storm in the next few days, but then the vertical shear is forecast to become fairly hostile after about 3-4 days, putting a brake on whatever intensification might be happening.  The next two names on the list are Patty and Rafael, and depending on the timing of this disturbance and the next one I'm going to talk about, this could be either.

The second disturbance has been a trackable entity since about October 6, and appears to be very close to Depression status (I think it has been a Depression for a while already, but NHC hasn't officially upgraded it).  However, it now has less than a day to "make its move" because a large frontal boundary is encroaching on it and will envelope it shortly.  On the large-scale water vapor image shown here, the disturbance in question is the blob of purples north of Hispaniola (the bright colors denote cold cloud tops and higher upper-level humidity, while the browns and blacks denote much lower humidity in the upper-levels).  Being that close to a mid-latitude trough/front is bad news if you're a tropical cyclone.

An enhanced satellite image with a closer view of the system shows the two air masses very clearly.  In the low-level clouds (yellow), two features are worth pointing out: 1) the surface Low is mostly exposed and centered to the southwest of the deep convection (white), and 2) the frontal boundary between the warm moist tropical air and the cooler drier mid-latitude air is evident and runs southwest to northeast... very close to the disturbance/depression.

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